Update docstring of tf.linespace for 0-D inputs

In 20258 the question was raised as tf.linespace
only supports 0-D inputs of start/stop, but the docstring
did not mention the 0-D restriction.

This fix updates the docstring to address the discrepancy.

This fix fixes 20258.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>